1. Field of the Invention
The present invention relates generally to holograms and the photosensitive emulsions that are present on holographic plates. More specifically, the invention relates to reducing the poor holographic image quality which results from uneven shrinkage of photosensitive gelatin during chemical development of the latent image contained in the gelatin.
2. Description of Related Art
Holograms are widely used for a variety of purposes including: helmet-mounted displays; eye protection reflectors for laser radiation; non-destructive structural testing of manufactured parts; and novelty displays. In fabricating a hologram, a holographic plate comprising a layer of photosensitive emulsion on a substrate is exposed to an electromagnetic wave interference pattern to record a latent image on the plate. The photosensitive emulsion is then developed by a series of chemical treatments to obtain the recorded latent image.
A photosensitive emulsion widely used in fabricating holograms is dichromated gelatin. A problem has been experienced with the use of dichromated gelatin as the photosensitive emulsion on holographic plates in that the chemical development of the latent image recorded in the gelatin results in uneven shrinkage of the gelatin, thereby producing a wavy surface on the holographic plate and a hologram of reduced optical image quality.
More specifically, when a holographic plate's coating of dichromated gelatin is exposed to interfering wave patterns of electromagnetic energy, the interference patterns induce energy into the gelatin. Where the interference pattern consists of constructively interfering waves, a greater amount of energy is imparted to the gelatin. Conversely, where destructively interfering patterns exist, a lesser amount of energy is imparted to the gelatin. The energy imparted to the gelatin initiates a chemical reaction which converts the ammonium dichromate to chromium ion and starts the polymerization of the gelatin. The degree of polymerization or cross-linking is proportional to the amount of energy imparted. Hence, in areas where constructively interfering wave patterns impart energy to the gelatin, the degree of cross-linking will be greater. And in areas where energy is imparted by destructively interfering wave patterns, the cross-linking will be less.
The existence of areas of varying degrees of cross-linking within the gelatin results in localized differences in the gelatin's molecular weight and refractive index: where cross-linking is greater, molecular weight and refractive index are greater; and where cross-linking is less, molecular weight and refractive index are less. When a holographic plate utilizing a dichromated gelatin photosensitive emulsion undergoes the chemical treatments required to develop a latent image recorded in the gelatin, the localized differences in molecular cross-linking and molecular weight result in uneven shrinkage of the gelatin. This in turn results in uneven lines in the interference pattern recorded in the gelatin and reduced optical image quality of the hologram.
The above-mentioned problem of uneven gel shrinkage is a particular problem in connection with slant holograms. The slant wave interference pattern in a slant hologram produces slanted layers in the gel having varying degrees of cross-linking. Variations in the shrinkage of the slanted layers during processing of the slant holograms produces wavy fringe patterns that adversely affect image quality.
In view of the above problems, there is a continuing need to provide improved photosensitive gels which can be developed according to conventional hologram processing procedures without problems of uneven shrinkage and the resulting poor image quality.